Previous studies on both normal and thyroid deficient rats (possessing a reduced intrinsic cardiac contractile function) show that they can improve their maximal exercise without undergoing changes in intrinsic cardiac contractile potential. These findings suggest that the heart may possess adequate contractile reserve to meet the demands of physical training. A series of experiments will now be carried out using rats to test the combined effects of chemical sympathectomy and adrenalmedullectomy on: (1) maximal heart rate and maximal exercise capacity (VmaxO2); (2) contractile function of the heart stressed in situ; (3) biochemical properties of the contractile system involving Ca ions regulation of myosin and of myofibril ATPase; and (4) adaptive capacity of the above parameters in response to physical training. Sympathectomy and adrenalmedullectomy will be performed to remove the primary neural-humoral stimulus known to enhance cardiac function during exercise. The experiments will provide insight regarding the acute and chronic adaptive capacity of the heart while functioning in the absence of its normal inotropic potential during exercise.